Surface plasmons are coherent electron oscillations that exist at the interface between any two materials where the real part of the dielectric function changes sign across the interface (e.g., a metal-dielectric interface). When surface plasmons couple with an incident photon, the resulting hybridized excitation is called a surface plasmon polariton (SPP). Therefore SPPs are bound electromagnetic modes that can occur at metal-dielectric interfaces and correspond to polarization waves of free electrons in the metal. Therefore, electromagnetic modes can propagate along a bare metal surface and decrease exponentially in the perpendicular direction, thereby strongly confining light to the surface until energy is lost either via absorption in the metal or radiation into free space.
An optical grating is formed by a periodically modulated interface between a dielectric and a metal. It is well known that at such structured surfaces, the absorption may strongly depend on the angle of incidence and the polarization of the incident light. Therefore, in angular reflectivity measurements, minima are observed at specific angles of incidence that are attributed to the incident light being transformed into a SPP wave which dissipates the energy in the metal, finally being converted to thermal energy. Conversely, if the thermal energy of the metal is high enough to populate surface plasmon states, the grating can transform these excitations into plane electromagnetic waves leaving the surface, leading to a maximum in emission observed near the angle at which the minimum of reflectivity is observed in the reflectivity measurement, as expected according to Kirchhoff's law of thermal radiation. Specifically, surface waves that are excited by heating the metal are diffracted by the grating in directions for which the condition of phase matching is met as a result of some spatial coherence in the surface plane. Therefore, the absorption and emission spectra can be highly directional and of a narrow bandwidth defined by the symmetry and periodicity of the surface structure for a given wavelength.
Surface plasmons in the mid-wave and long-wave infrared (LWIR), nominally 3-15 μm, have notable differences to plasmons in the visible and near-infrared part of the electromagnetic spectrum. Two defining characteristics of SPP modes are the propagation length, δspp, and the dielectric surface penetration depth, kd. These features are directly proportional to one another over all frequency ranges, but exist in opposite extremes for the visible and infrared (IR), with propagation length and dielectric penetration being small in comparison to wavelength in the visible, and quite large in the infrared. These physical properties of the LWIR SPP's have made it a challenge to find efficient applications in this frequency range, though some have been demonstrated. See C. Sirtori et al., Opt. Lett. 23, 1366 (1998); and N. Yu et al., Nature Photonics 2, 564 (2008). For example, performance of the quantum cascade laser (QCL) has been improved through the use of SPP waveguide modes, while the beam profile and polarization state of mid-IR QCL's have been modified using SPP's as well.
However, for further applications to be developed, a need remains for long-wavelength plasmons that can be generated with the high confinement and high loss of conventional SPP's. One potential application of long-wavelength plasmons is directional emission, where plasmon radiation loss can be beneficial. Careful design of a periodic surface structure can allow the emission (absorption) to occur only over a narrow angular range.